crust-mantle interactions and generation of silicic …and tectonic aspects of bimodal magmatism and...
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Andean Geology
ISSN: 0718-7092
Servicio Nacional de Geología y Minería
Chile
Calderón, Mauricio; Hervé, Francisco; Cordani, Umberto; Massonne, Hans-Joachim
Crust-mantle interactions and generation of silicic melts: insights from the Sarmiento Complex,
southern Patagonian Andes
Andean Geology, vol. 34, núm. 2, julio, 2007, pp. 249-275
Servicio Nacional de Geología y Minería
Santiago, Chile
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Crust-mantle interactions and generation of silicic melts:
insights from the Sarmiento Complex,
southern Patagonian Andes
Mauricio CalderónFrancisco Hervé
Umberto Cordani
Hans-Joachim Massonne
Departamento de Geología, Universidad de Chile,Casilla 13518, Correo 21,Santiago, Chile
[email protected]@cec.uchile.cl
Centro de Pesquisas Geocronológicas, Universidade de São Paulo,Rua do Lago 562, Cidade Universitaria, São Paulo, CEP 05508-900, Brazil
Institut für Mineralogie und Kristallchemie, Universität Stuttgart,Azenbergstrasse 18, D-70174, Stuttgart, Germany
ABSTRACT
A Late Jurassic seafloor remnant of the Rocas Verdes basin in southern Chile, the Sarmiento Complex (ca. 52°S),bears lithological layers with bimodal meta-igneous rocks appropriate for a comprehensive investigation of magmagenesis in part of a lateral lithological transition from continental rifting to initial seafloor spreading. Metamorphosed maficrocks collected from different layers in the ophiolite pseudostratigraphy and a plagiogranite have positive εNd150 values(+ 1 and +2). Granophyres, which are crosscut by ophiolitic mafic dikes, have negative εNd
150 values (-5). Dacitic dikes
within thick successions of pillow basalt have the least negative εNd150 values (-3 and -4). Although mafic and felsicigneous rocks show contrasting isotopic signatures, thermochemical modeling (EC-AFC) suggests they can share acommon origin. Models consider an arbitrary composition of the crustal assimilant (mostly metapelite with an averageεNd
150 value of -7) and evaluate the feasibility for generation of silicic melts through the interaction of mafic magmas and
metasedimentary rocks. A quantitative evaluation of basaltic magma contaminated by crustal wall rocks requires a Ma*/Mc (mass of anatectic melt/ mass of cumulates) ratio of 0.04. Analyses using dikes of dacite (with Ma*/Mc ratios rangingbetween 0.28-0.35) and granophyres (with Ma*/Mc ratios of 0.63-0.89) require the silicic magmas to contain higherproportions of anatectic melts derived from metamorphic rocks. Isotopic differences among granophyres and dacitescould be controlled by eruption dynamics, regional stress field and/or differences in thermal regimes in magmachambers. Bimodal magmatism in the earliest tectonic evolution of the Rocas Verdes basin could reflect regimes of slowextension of the continental crust along the Jurassic southwestern margin of Gondwana.
Key words: Rocas Verdes basin, Sarmiento Complex, Bimodal magmatism, Petrogenesis, Patagonia, Chile.
RESUMEN
Interacciones corteza-manto y generación de fundidos silíceos: perspectivas desde el ComplejoSarmiento, Andes patagónicos australes. Un remanente Jurásico del fondo oceánico de la cuenca de RocasVerdes, el Complejo Sarmiento (ca. 52°S), contiene niveles con asociaciones de rocas bimodales apropiadas para la
Revista Geológica de Chile, Vol. 34, No. 2, p. 249-275, 11 Figs., 6 tables, July 2007.
250 CRUST-MANTLE INTERACTIONS AND GENERATION OF SILICIC MELTS: INSIGHTS FROM THE SARMIENTO COMPLEX...
investigación de la génesis de magmas en parte de la transición litológica entre ‘rifting’ continental y etapas tempranasde la extensión oceánica. Rocas máficas metamorfizadas y plagiogranitos en distintos niveles de la pseudoestratigrafíaofiolítica, tienen valores positivos de εNd150 (+1 and +2). Granófiros, que se encuentran atravesados por diques máficoscon afinidades ofiolíticas, tienen valores negativos de εNd
150 (-5). Diques de dacita que intruyen una sucesión de basaltos
almohadillados también presentan valores de εNd150 negativos (-3 y -4). Aunque las rocas estudiadas muestran razonesisotópicas contrastantes, un modelo termoquímico, energéticamente balanceado (EC-AFC) sugiere que ellas puedencompartir un origen en común. Los modelos consideran una composición arbitraria de asimilante cortical (principalmentemetapelita con un valor promedio de εNd
150 cercano a -7) y evalúa la posibilidad de generación de magmas silíceos a
través de la interacción de magmas máficos y rocas metasedimentarias. Una evaluación cuantitativa de lacontaminación de magma basáltico por rocas encajantes, indica razones de Ma*/Mc (masa de líquido anatéctico/ masade cumulados) de 0,04. En cambio, análisis cuantitativos en diques de dacita (con Ma*/Mc entre 0,28-0,35) y granófiros(con Ma*/Mc entre 0,63-0,89), indican que magmas silíceos contienen mayor proporción de líquidos antécticosderivados de rocas metamórficas. Las diferencias isotópicas entre granófiros y dacitas podrían haber sido controladaspor la dinámica de la erupción, régimen de esfuerzos tectónicos y/o por diferencias de regímenes termales en la cámaramagmática. El magmatismo bimodal en las etapas tempranas de la evolución de la cuenca de Rocas Verdes,probablemente refleja regímenes con bajas tasas de extensión en la corteza continental del margen sudoccidental deGondwana durante el Jurásico.
Palabras Claves: Cuenca Rocas Verdes, Complejo Sarmiento, Magmatismo bimodal, Petrogénesis, Patagonia, Chile.
INTRODUCTION
Crust-mantle interactions involve processes ofmass, chemical and thermal exchange betweenmantle-derived basaltic magmas and crustalmetamorphic rocks. Basaltic injection into the crusthas been widely considered as an importantmechanism to generate silicic melts in the con-tinental and oceanic crust (e.g., Huppert and Sparks,1988; Pankhurst and Rapela, 1995; Petford andGallagher, 2001). Basalt can act either as a parentalsource or as the thermal trigger for crustal melting(Grunder, 1995). The latter process is controlled bythe fertility of the crustal material, temperature andthe water content of basaltic magma (Annen andSparks, 2002). Evidences from field and geo-chemical studies and experimental petrology pointto amphibolite, metapelite and metagreywacke assuitable crustal lithologies for partial melting andgeneration of silicic melts (e.g., Wolf and Wyllie,1994; Rapp and Watson, 1995; Patiño Douce andBeard, 1995; Milord et al., 2001). Even partial meltingexperiments performed with mixtures of dry basaltand metapelite yielded water-undersatured graniticmelts (Patiño Douce, 1995) at temperatures of1,000°C and 5-15 kbar pressure.
Several ophiolite complexes and overlyinghemipelagic sedimentary successions exposedalong the southern Pacific margin of South America(Fig. 1) have been considered as seafloor remnants
of the Late Jurassic-Early Cretaceous Rocas Verdesbasin (Katz, 1964; Dalziel et al., 1974; Suárez andPettigrew, 1976; Fuenzalida and Covacevich, 1988;Stern and de Wit, 2003). The Sarmiento Complexpreserves a lithological pseudostratigraphy withmixed mafic-felsic layers and without the ultramaficcomponents of ‘classical’ ophiolites. De Wit andStern (1981) argued for its development over adiffuse zone, dominated by the interaction betweenmafic magmas and continental rocks, when twocontinental blocks were separated. SHRIMP U-Pbzircon geochronology constrain bimodal volcanismto the time from 152 to 147 Ma (Calderón, 2006;Calderón et al., in press) enhancing the SarmientoComplex as a geological setting where petrologicaland tectonic aspects of bimodal magmatism andcontinental lithosphere rupturing can be studied.For this purpose, whole rock Sr and Nd isotopicratios, mineral chemistry and 40Ar/39Ar ages havebeen obtained in an appropriate suite of rockscollected over several field seasons. The use of Ndisotopic ratios and the thermodynamic frameworkof the energy-constrained assimilation fractional-crystallization (EC-AFC) model for complex openmagmatic systems (Spera and Bohrson, 2001)provide petrological and thermochemical insightsinto the generation of silicic magmas through theinteraction of basalt and fertile metamorphic rocks.
M. Calderón, F. Hervé, U. Cordani, H-J. Massonne 251
FIG. 1. Location map of the geological units in the southwestern Patagonian and Fuegian Andes.
GEOLOGICAL BACKGROUND
The Sarmiento Complex formed as part of asubmarine and partitioned basin that was con-tinuously filled with the hemipelagic sedimentarysuccessions known as the 'black shales' of theZapata Formation (Allen, 1982; Fildani and Hessler,2005). Basin development involved widespreaderuptions of subaerial and subaquatic felsicignimbrites that were deposited in the fault-boundedgrabens of the basin associated with the ElQuemado Complex (ca. 155 Ma; Pankhurst et al.,2000) and the Tobífera Formation (ca. 148-142 Ma;Calderón et al., in press). The Rocas Verdes basinevolved to a backarc basin in Early Cretaceoustimes (Wilson, 1991; Calderón, 2006) when rhythmicsuccessions of fine grained distal turbidites in theupper member of the Zapata Formation (cf.Fuenzalida and Covacevich, 1988) were deposited.Basin closure and obduction onto the cratonicmargin occurred in the mid-Cretaceous (Dalziel,1981) before the main orogenic deformation andrapid uplift of the Andean mountain chain leading
to the initiation of the Magallanes foreland basin at~92 Ma (Fildani et al., 2003).
The formation of the Sarmiento Complex wascontemporaneous in part with the emplacement of theLate Jurassic and eastern plutonic belt of the SouthPatagonian batholith (Fig.1), which is mainly composedof leucogranite with some gabbro (Hervé et al., 2007).Narrow and discontinuous belts of andalusite andsillimanite gneisses as well as migmatites occur inclose spatial relation to the granitic belt (Watters, 1964;Calderón, 2005). A Late Jurassic age for crustalanatexis has been established through dating ofovergrowth zones of zircons from the sillimanitegneisses (Hervé et al., 2003). At the Puerto EdénIgneous and Metamorphic Complex (Fig. 1), granitesare considered as mixtures of variable amounts ofradiogenic crust-derived anatectic melts and lessradiogenic mafic magmas (Calderón et al., 2007).
Late Paleozoic polydeformed metasedimentaryrocks, grouped in the Staines Complex and theEastern Andean Metamorphic Complex (Forsythe
252 CRUST-MANTLE INTERACTIONS AND GENERATION OF SILICIC MELTS: INSIGHTS FROM THE SARMIENTO COMPLEX...
and Allen, 1980; Hervé et al., 1998; Hervé et al.,2003; Augustsson, 2003; Augustsson et al., 2006),crop out to the east of the South Patagonian batholith(Stewart et al., 1971)1. To the west of the batholith (atArchipiélago Madre de Dios; Fig. 1) thick successionsof limestone, basalt and chert are thought to representfragments of seamounts accreted to the paleo-Pacificmargin of Gondwana (Forsythe and Mpodozis, 1983)between Early Permian and Early Jurassic times(Thomson and Hervé, 2002). The turbidites of theDuque de York Complex were deposited on top of
the limestone- and basalt- successions betweenpost-Early Permian to pre-Early Jurassic times in anactive margin setting (Faúndez et al., 2002; Lacassieet al., 2006). The Diego de Almagro MetamorphicComplex, that crops out to the west and is separatedfrom the Duque de York complex by the northwest-southeast trending Seno Arcabuz shear zone(Olivares et al., 2003), consists of foliated meta-granite, paraschists and blueschists juxtaposed in asubduction zone setting in the Early Cretaceous(Willner et al., 2004).
OCURRENCE AND MINERALOGY OF STUDIED SAMPLES
Samples for this study were collected from thepre-Jurassic metasedimentary rocks of the StainesComplex, the bimodal meta-igneous rocks of theSarmiento Complex, felsic metamorphosed tuffs ofthe Tobífera Formation and late small volumeigneous rocks in the Sarmiento Complex.
STAINES COMPLEX
Greenschist facies psammopelitic schists of theStaines Complex (Stewart et al., 19711; Forsythe andAllen, 1980; Allen, 1982; Hervé et al., 2003) crop outto the west of the Sarmiento Complex (Fig. 2). Theyconsist of fine-grained and polydeformed white-micachlorite schists with folded and crenulated millimeter-to centimeter-wide bands of granoblastic quartz andrestricted plagioclase. Minor amounts of chloritizedblasts of biotite also occur. Tourmaline, zircon, apatiteand opaque such as graphite and Fe-Ti oxides areaccessory phases. The turbiditic protolith of meta-morphic rocks was deposited in the late Paleozoic(Hervé et al., 2003; Augustsson et al., 2006).
SARMIENTO COMPLEX
In the southern segment of the Sarmiento Complexthe incomplete ophiolite pseudostratigraphy dipsgently to the south. Metagabbros (sills and dikes)and plagiogranites occur in the northern parts of thecomplex (Seno Lolos), whereas extrusive rocksdominate the southern exposures to Canal Unión(Fig. 2). Field work in the Sarmiento Complex hasshown that it is possible to distinguish three mainlithological layers in the region, as discussed below:
1. A mafic extrusive layer: exposed along theCordillera Sarmiento is composed of a kilometer-thick, flat-lying succession of pillow and massivebasalt, pillow breccias, and restricted intercalationsof radiolaria-bearing cherts and siltstones. Olivinebasalts show porphyritic and branching textureswith metamorphic assemblages consisting ofpumpellyite, chlorite, quartz, plagioclase, epidote,titanite, carbonate and rare actinolite. Metamorphicphases occur in amygdules, phenocrysts, veins andthe groundmass. Olivine is totally replaced by calciteand/or chlorite. Clinopyroxene is the only relict maficigneous phase. Meter-wide dikes of fine graineddolerite composed mainly of subhedral plagioclase(35-45%), ophitic clinopyroxene (25-35%), inter-stitial chlorite (13-23%) and accessory quartz,titanite and epidote occur in this layer.
2. A mafic-felsic extrusive layer: exposed alongPenínsula Taraba and at Isla Young consists mainlyof successions of pillow basalts with intercalationsof silicic tuffs and hyaloclastites. These successionsare cut by late meter-wide dikes of dacite and rhyolite.Porphyritic and amygdaloidal basalts show variableamounts (5-15%) of olivine phenocrysts that aretotally retrogressed to chlorite. In some cases, theycontain calcite. A branching texture in thegroundmass is defined by an aggregate ofplagioclase, clinopyroxene, interstitial chlorite,accessory Fe-Ti oxides and pyrite. Felsic dikes areporphyritic rocks with phenocrysts of embayed quartzand subhedral plagioclase, and accessory allaniteand chloritized pseudomorphs. Some of these rocksare partially brecciated and have myarolitic cavitiessuggesting hypabyssal conditions during crystalliza-
1 Stewart, J.; Cruzat, A.; Page, B.; Suárez, M.; Stambuk, V. 1971. Estudio geológico económico de la Cordillera Patagónica entre los paralelos 51°00" y 53°30" Lat.S,provincia de Magallanes. Instituto de Investigaciones Geológicas (Inédito): 174. Santiago, Chile.
M. Calderón, F. Hervé, U. Cordani, H-J. Massonne 253
tion. The quarzofeldspathic and spherulitic groun-dmass is aphanitic. Apatite and zircon are accessory
FIG. 2. Geological map of the Cordillera Sarmiento and surrounding areas (modified from Allen 1982 and Stewart et al., 19711). Main localitiesand the location of samples are shown. If more than one sample was collected in a single locality, numbers are followed by capitalletters.
phases. The secondary assemblage consists ofchlorite, epidote, quartz and feldspars.
254 CRUST-MANTLE INTERACTIONS AND GENERATION OF SILICIC MELTS: INSIGHTS FROM THE SARMIENTO COMPLEX...
The mafic-felsic extrusive layer includes a narrowbelt of sheared felsic and mafic meta-igneous rockswith a north-northwest trending subvertical foliation.The metamorphic assemblage in strain fringes andother microstructures in mafic rocks consists ofactinolite, quartz, plagioclase, chlorite and titanite. Inthe foliated felsic rocks, the predominant metamorphicphases are chlorite, epidote, titanite, quartz andfeldspar with minor amounts of white mica. At thesouthern tip of Península Taraba (Bahía Intricate; Fig.2), a small pluton of fine-grained biotite tonalite is incontact with massive, hornfelsic basalts and the foldedsedimentary successions of the Zapata Formation.The biotite tonalite contains microscopic inclusions ofmetamorphic rocks composed of quartz, plagioclaseand decussate biotite.
3. A mafic-felsic intrusive layer: exposedbetween the east-west trending Seno Lolos, SenoEncuentros and along the northern shore of SenoBenavente (Fig. 2), consists of fine to medium-grained plagioclase- and quartz-rich porphyriticrocks with granophyric and graphic textures, here-inafter called granophyres. They are characterizedby the presence of accessory allanite with narrowand irregular rims of colorless epidote. Thegranophyres are intruded by a swarm of north-southtrending subvertical dike complexes of coarse- andfine-grained metagabbro. The dikes have inter-granular textures with no preserved pyroxene.Although most mafic dikes are straight and showcentimeter-wide chilled margins, some sinusoidaldikes occur. Sheeted dike complexes occur in highertopographic levels (deWit and Stern, 1981). Rarefine-grained plagiogranite dikes are in subhorizontalcontact with subvertical mafic dikes. The plagiogranitedikes are composed of plagioclase, quartz andmetamorphic epidote and chlorite. At the base of themafic-felsic intrusive layer, homogeneous medium-grained and 'amphibolitized' gabbroic sills or dikesare found that preserve intergranular and cumulatetextures that are defined by interlocking tabularplagioclase and xenomorphic hornblende. Interstitialopaque minerals are Fe-Ti oxides and pyrite. Sills ofhornblende tonalite with inclusions of amphiboliteand mafic aphanitic rocks appear at the base of thislayer. The amphibolites have granoblastic textureswith amphibole and plagioclase (melanosome) andleucocratic veins of tonalite (leucosome). Horn-blende grains in the melanosomes have scarcerelict inclusions of clinopyroxene. Hornblende in theamphibolites is retrogressed to actinolite, chlorite,
titanite, epidote and traces of biotite. Plagioclase,in some cases, is replaced by microcrystals of whitemica.
LATE SMALL VOLUME HORNBLENDE-BEARING
INTERMEDIATE ROCKS
Plutons of medium-grained and panhypidio-morphic hornblende tonalite with marginal gra-nodiorite components crop out at Seno Encuentro(Fig. 2). Porphyritic and medium-grained hornblendedacitic dikes, with subhedral phenocrysts ofhornblende, plagioclase and quartz within an afaniticquartzofeldspathic groundmass occur at SenoBenavente. The hornblende in them is partiallyreplaced by chlorite, epidote and carbonate.Plagioclase shows variable degree of sericitization.
TOBÍFERA FORMATION
Along the eastern flank of the Cordillera Sarmientoand the main part of the Cordillera Riesco, a kilometer-wide, north-south trending belt of sheared silicicpyroclastic rocks intercalated with meter-sizedhorizons of sheared quartz-bearing siltstones hasbeen mapped in the Tobífera Formation (Stewart etal., 19711; Bruhn et al., 1978; Allen, 1982; Galaz et al.,2005). These meta-rhyolites have crystallization agesof ca. 148 Ma (SHRIMP U-Pb zircon ages; Calderón,2006). Geothermobarometry of syntectonic meta-morphic assemblages consisting of phengite,chlorite, stilpnomelane, and quartz indicate mediumpressure greenschist facies conditions duringdynamic metamorphism (Hervé et al., 2004; Calde-rón et al., 2005). The sheared belt is intruded bymeter-wide, north-south trending sills of medium-grained chloritized dolerite, that (in studied samples)show no evidence of dynamic recrystallization.Consequently, the injection of the mafic magmas inthis belt occurred after shearing and, is not relatedto the ca. 150 Ma old mafic magmatic events.
Tobífera rocks crop out to the west and in thrustcontact over the Sarmiento Complex. They sharean unconformable depositional contact withPaleozoic metasedimentary rocks that are basalbreccias and conglomerates intercalated withgreywackes, sandstones, siltstones. Rhyoliticcrystal lapilli tuffs are dominant in the upper part ofthe succession (Allen, 1982). Minor lithic fragmentsof metasedimentary rocks and porphyritic rocksthat are common constituents in these pyroclastic
M. Calderón, F. Hervé, U. Cordani, H-J. Massonne 255
rocks show discrete stylolitic bands and anasto-mosing cleavages indicating dynamic recrystalliza-tion that is less intense than in the eastern sheared
belt. White mica, quartz and plagioclase constitutethe metamorphic assemblage. Pyrite is a frequentopaque phase.
MINERAL PHASE CHEMISTRY OF METAMORPHOSED MAFIC ROCKS
Mineral compositions were measured at theInstitut für Mineralogie und Kristallchemie, Uni-versität Stuttgart, using a CAMECA SX-100 micro-probe with 5 wavelength-dispersive systems.Operating conditions were: 15kV and 15 nA, abeam size of 7-10 µm or a focussed beam (for verysmall crystals), 20 seconds counting time on thepeak and on the background for each element. Thestandards used were natural wollastonite (Si, Ca),natural orthoclase (K), natural albite (Na), naturalrhodonite (Mn), synthetic Cr2O3 (Cr), synthetic TiO2
(Ti), natural hematite (Fe), natural baryte (Ba),synthetic MgO (Mg), synthetic Al2O3 (Al) andsynthetic NiO (Ni). The PaP correction procedureprovided by Cameca was applied.
The analyses were chosen to illustrate thecompositional ranges of pyroxene, plagioclase andamphibole found in the metamorphosed maficrocks. Studied samples were two medium-grainedamphibolitized metagabbros (samples ST0228and ST0229A) collected from the mafic-felsicintrusive layer. An amphibolite, consisting of anamphibole-rich melanosome and a quartz-plagio-clase leucosome was also analyzed (sampleST0226A). A fine-grained and weakly foliated dikeof metabasaltic-andesite (ST0204) and an olivinemetabasalt (OW9962) with lath-shaped crystals ofplagioclase and needles of clinopyroxene in thegroundmass were collected from the mafic-felsicextrusive layer. Mineral compositions are presentedin tables 1-3. The location of studied samples isillustrated in figure 2.
PYROXENE
Partially affected by metamorphic processes,clinopyroxene, which is predominantly augite incomposition (Fig. 3), shows variable Mg-number[(100*Mg/ (Mg+Fe2+)] and minor element concentra-tions. No compositional variation was detectedwhere relict crystals allowed core and rim domainsto be analyzed (see analyses of samples ST0204and ST0228 in table 1). Clinopyroxene is the main
stable igneous phase in the mafic rocks and itschemistry reflects primary magmatic compositions.
The most primitive pyroxene grains, found inmetagabbro ST0228, have Mg-numbers rangingfrom 80 to 90 in most cases and Cr2O3 contents ashigh as 0.36 wt% (Fig. 4a). Lower Mg-numbers inpyroxenes in the other metagabbro ST0229Asuggest crystallization from a more fractionated mag-ma. The clinopyroxene in both the dikes and lavashave Mg-number that range from approximately 67to 75. Metabasalt OW9962 has clinopyroxene withconspicuously higher contents of TiO2 and Al2O3,and elevated Na2O contents relative to otherlithologies (Figs. 4b-d). These clinopyroxene crystalsalso have higher Cr2O3 contents than those in themetabasaltic-andesite and the evolved metagabbro.Although a negligible acmite component is found inthe clinopyroxene, sodium contents show a goodcorrelation with Mg-number (Fig. 4d).
PLAGIOCLASE
All of the plagioclase in the metabasalts andmetabasaltic-andesites is albite in composition(Fig. 5) indicating a metamorphic origin. The albite
FIG. 3. Pyroxene quadrilateral plots for the ophiolitic meta-morphosed mafic rocks of the Sarmiento Complex.
Augite
HdDi
En Fs
MetabasaltMetabasaltic-andesite (dike)
Metagabbros
256 CRUST-MANTLE INTERACTIONS AND GENERATION OF SILICIC MELTS: INSIGHTS FROM THE SARMIENTO COMPLEX...
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0.36
0.25
0.23
0.35
0.23
0.27
0.31
0.28
0.19
0.21
0.23
0.26
0.16
0.26
0.32
0.32
0.31
0.32
0.35
K 2O0.
000.
010.
010.
020.
010.
000.
010.
010.
000.
020.
020.
030.
000.
000.
000.
000.
010.
010.
010.
000.
010.
05
NiO
0.05
0.01
0.04
0.00
0.00
0.00
0.01
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
Tota
l10
0.15
99.9
999
.45
99.9
998
.64
98.4
498
.69
99.0
699
.04
98.7
699
.38
99.3
299
.62
99.6
699
.68
98.7
798
.95
98.7
699
.37
99.2
298
.87
98.1
2
Si1.
749
1.75
41.
749
1.72
01.
898
1.89
41.
886
1.89
21.
898
1.90
71.
912
1.94
11.
919
1.89
91.
907
1.91
21.
883
1.87
31.
939
1.94
61.
913
1.90
4
Aliv
0.25
10.
233
0.25
10.
280
0.07
60.
088
0.11
20.
086
0.10
20.
093
0.08
80.
046
0.08
10.
101
0.08
50.
079
0.11
30.
106
0.05
00.
036
0.07
50.
085
Ti0.
073
0.07
70.
079
0.08
10.
021
0.02
40.
024
0.02
40.
025
0.02
10.
023
0.00
90.
013
0.01
20.
017
0.01
50.
027
0.02
60.
008
0.00
70.
015
0.01
8
Alv
i0.
026
0.00
00.
004
0.02
70.
000
0.00
00.
000
0.00
00.
000
0.01
50.
003
0.00
00.
022
0.00
80.
000
0.00
00.
000
0.00
00.
000
0.00
00.
000
0.00
0
Fe3+
0.10
00.
125
0.10
70.
111
0.10
50.
094
0.09
50.
101
0.07
30.
060
0.06
00.
064
0.04
20.
075
0.08
20.
070
0.08
30.
119
0.08
00.
080
0.09
10.
098
Cr
0.00
40.
006
0.00
60.
006
0.00
00.
000
0.00
00.
000
0.00
00.
001
0.00
00.
005
0.00
60.
010
0.00
40.
009
0.00
40.
001
0.00
10.
000
0.00
10.
001
Fe2+
0.27
50.
311
0.27
90.
262
0.35
30.
314
0.28
20.
309
0.30
60.
322
0.31
30.
139
0.15
80.
119
0.17
00.
133
0.20
00.
158
0.26
00.
294
0.24
90.
227
Mn
0.00
80.
012
0.00
80.
008
0.01
10.
011
0.01
10.
010
0.01
00.
009
0.01
00.
008
0.00
50.
005
0.00
60.
006
0.00
90.
009
0.00
90.
010
0.01
00.
010
Mg
0.62
30.
631
0.65
00.
647
0.72
30.
798
0.80
30.
815
0.82
40.
803
0.80
40.
858
0.91
80.
923
0.85
40.
906
0.84
40.
813
0.77
40.
742
0.77
60.
786
Ca
0.86
90.
827
0.84
30.
835
0.79
80.
763
0.76
50.
750
0.74
50.
749
0.76
70.
915
0.82
20.
832
0.85
80.
860
0.82
00.
875
0.85
80.
864
0.84
90.
845
Na
0.02
40.
027
0.02
40.
026
0.01
90.
017
0.02
50.
017
0.02
00.
023
0.02
00.
014
0.01
50.
017
0.01
90.
012
0.01
90.
024
0.02
40.
023
0.02
40.
026
K0.
000
0.00
00.
000
0.00
10.
000
0.00
00.
000
0.00
00.
000
0.00
10.
001
0.00
10.
000
0.00
00.
000
0.00
00.
001
0.00
00.
000
0.00
00.
000
0.00
3
Ni
0.00
10.
000
0.00
10.
000
0.00
00.
000
0.00
00.
000
0.00
00.
000
0.00
00.
000
0.00
00.
000
0.00
00.
000
0.00
00.
000
0.00
00.
000
0.00
00.
000
Tota
l4.
003
4.00
54.
003
4.00
34.
004
4.00
34.
003
4.00
34.
002
4.00
24.
002
4.00
14.
001
4.00
14.
002
4.00
14.
002
4.00
34.
002
4.00
24.
003
4.00
3
Mg-
No.
6967
7071
6772
7473
7371
7286
8589
8387
8184
7572
7678
[mg]
=100
*Mg/
(Mg+
Fe2+
)
TABLE 2. REPRESENTATIVE CHEMICAL ANALYSES OF PLAGIOCLASE IN METAMORPHOSED MAFIC ROCKS (O=8).
Sample OW9962 (metabasalt) ST0204 (metabasaltic-andesite) ST0228 (metagabbro) ST0229A (metagabbro) ST0226A (melanosome in amphibolite) ST0226A (leucosome in amphibolite)
Spot 1 2 3 71 10 14 5 79 18 40 23 65 12 46 16 3 48 103 141 27 82 110 118
SiO2 68.43 68.87 68.63 68.59 68.85 69.07 69.29 52.43 46.75 46.59 46.07 45.69 64.17 55.80 51.94 51.62 50.40 67.49 62.00 58.11 55.73 52.37 51.18TiO2 0.00 0.00 0.01 0.01 0.03 0.01 0.01 0.06 0.03 0.03 0.02 0.03 0.03 0.03 0.06 0.07 0.05 0.01 0.02 0.01 0.02 0.05 0.02Al2O3 20.56 20.00 19.95 21.13 20.94 21.23 21.30 29.24 32.55 32.58 33.23 33.67 23.66 27.46 29.29 29.83 30.52 22.41 23.61 26.68 27.03 25.45 30.38Cr2O3 0.00 0.04 0.03 0.00 0.00 0.00 0.01 0.00 0.01 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.01 0.00 0.02 0.00FeO 0.18 0.16 0.12 0.15 0.20 0.33 0.37 0.46 0.71 0.67 0.62 0.63 0.25 0.44 0.56 0.56 0.53 0.08 0.10 0.28 0.22 2.17 0.22MnO 0.04 0.02 0.04 0.00 0.01 0.01 0.00 0.00 0.02 0.00 0.01 0.00 0.03 0.00 0.00 0.00 0.01 0.01 0.00 0.00 0.00 0.00 0.00MgO 0.04 0.03 0.02 0.00 0.00 0.00 0.00 0.05 0.04 0.04 0.02 0.02 0.01 0.06 0.05 0.09 0.03 0.16 0.00 0.02 0.01 0.02 0.01CaO 0.41 0.49 0.12 0.43 0.39 0.44 0.71 12.27 16.33 16.62 17.09 17.69 3.87 9.77 12.81 12.89 13.88 0.85 4.13 8.44 9.27 12.63 13.28Na2O 11.42 11.58 12.14 11.04 11.27 11.42 10.09 4.86 2.39 2.28 2.04 1.65 9.06 6.25 4.50 4.59 4.04 9.83 7.63 7.08 6.58 4.35 4.14K2O 0.39 0.16 0.09 0.10 0.05 0.08 0.06 0.19 0.05 0.09 0.06 0.05 0.13 0.12 0.08 0.16 0.09 0.66 0.08 0.08 0.09 0.04 0.05Total 101.49 101.36 101.14 101.46 101.73 102.59 101.85 99.56 98.89 98.91 99.17 99.43 101.20 99.92 99.27 99.81 99.57 101.50 97.55 100.70 98.94 97.09 99.28
Si 2.955 2.975 2.973 2.952 2.956 2.946 2.962 2.396 2.179 2.173 2.145 2.124 2.798 2.519 2.382 2.359 2.315 2.906 2.791 2.588 2.536 2.473 2.346Ti 0.000 0.000 0.000 0.000 0.001 0.000 0.000 0.002 0.001 0.001 0.001 0.001 0.001 0.001 0.002 0.002 0.002 0.000 0.001 0.000 0.001 0.002 0.001Al 1.047 1.018 1.018 1.072 1.060 1.067 1.073 1.575 1.788 1.791 1.824 1.845 1.216 1.461 1.583 1.607 1.652 1.137 1.253 1.401 1.449 1.416 1.641Cr 0.000 0.001 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.001 0.000Fe 0.007 0.006 0.004 0.005 0.007 0.012 0.013 0.018 0.028 0.026 0.024 0.024 0.009 0.017 0.021 0.021 0.021 0.003 0.004 0.010 0.009 0.086 0.009Mn 0.002 0.001 0.001 0.000 0.000 0.000 0.000 0.000 0.001 0.000 0.000 0.000 0.001 0.000 0.000 0.000 0.001 0.000 0.000 0.000 0.000 0.000 0.000Mg 0.002 0.002 0.001 0.000 0.000 0.000 0.000 0.004 0.003 0.002 0.002 0.002 0.000 0.004 0.003 0.006 0.002 0.011 0.000 0.001 0.000 0.001 0.000Ca 0.019 0.023 0.006 0.020 0.018 0.020 0.033 0.601 0.815 0.831 0.853 0.881 0.181 0.472 0.629 0.631 0.683 0.039 0.199 0.403 0.452 0.639 0.652Na 0.956 0.970 1.020 0.921 0.938 0.944 0.836 0.430 0.216 0.206 0.184 0.149 0.766 0.547 0.400 0.407 0.360 0.821 0.666 0.612 0.580 0.398 0.368K 0.022 0.009 0.005 0.005 0.002 0.004 0.003 0.011 0.003 0.005 0.004 0.003 0.007 0.007 0.005 0.010 0.005 0.036 0.004 0.004 0.005 0.002 0.003Total 5.010 5.005 5.030 4.975 4.983 4.994 4.921 5.036 5.035 5.036 5.036 5.028 4.979 5.027 5.026 5.043 5.040 4.953 4.917 5.019 5.032 5.017 5.019
An 1.92 2.26 0.55 2.10 1.85 2.08 3.74 57.64 78.79 79.68 81.95 85.33 18.96 46.04 60.88 60.26 65.16 4.35 22.90 39.55 43.56 61.49 63.75Ab 95.91 96.88 98.98 97.34 97.89 97.49 95.86 41.31 20.90 19.80 17.68 14.41 80.28 53.30 38.67 38.83 34.36 91.61 76.59 60.03 55.94 38.29 35.96Or 2.17 0.86 0.47 0.57 0.26 0.43 0.40 1.06 0.31 0.51 0.37 0.26 0.76 0.65 0.45 0.91 0.48 4.04 0.51 0.42 0.50 0.21 0.29
56 60 63 57
64.13 59.27 58.80 58.740.01 0.03 0.04 0.0323.24 25.54 25.53 25.910.01 0.00 0.00 0.020.26 0.35 0.20 0.460.02 0.01 0.00 0.000.00 0.00 0.00 0.013.46 6.98 7.06 7.439.54 7.88 7.70 7.530.31 0.16 0.15 0.18
100.97 100.22 99.49 100.30
2.807 2.645 2.641 2.6240.000 0.001 0.001 0.0011.199 1.343 1.352 1.3640.000 0.000 0.000 0.0010.010 0.013 0.008 0.0170.001 0.000 0.000 0.0000.000 0.000 0.000 0.0000.162 0.334 0.340 0.3560.809 0.682 0.671 0.6520.017 0.009 0.008 0.0105.006 5.028 5.021 5.024
16.42 32.55 33.34 34.9381.83 66.55 65.83 64.041.76 0.91 0.83 1.03
TABLE 3. REPRESENTATIVE CHEMICAL ANALYSES OF AMPHIBOLE IN METAMORPHOSED MAFIC ROCKS (O=23).
Sample ST0204 (metabasaltic-andesite) ST0228 (metagabbro) STO229A (metagabbro) ST0226A (melanosome in amphibolite)Spot Nª 13 28 30 37 39 15 2 6 83 61 51 56 11 41 84 23 10 36 52 50 30 32 33 107 87 136 14 19 127 96 26 80 79 84 133
SiO2 48.43 49.92 50.28 53.12 53.84 55.36 54.92 53.87 53.87 53.82 53.65 53.02 52.72 51.46 50.90 54.04 53.73 52.50 52.25 51.41 50.31 50.15 48.40 48.28 47.52 47.18 47.11 47.07 46.88 46.41 46.40 46.35 45.55 45.27 45.11
TiO2 0.04 0.02 0.03 0.01 0.02 0.20 0.09 0.16 0.23 0.12 0.12 0.16 0.18 0.23 0.74 0.11 0.07 0.22 0.15 0.95 0.20 0.25 0.85 1.14 1.45 1.43 1.42 1.47 1.55 1.55 1.78 1.62 1.73 1.67 1.73
Al2O3 4.03 2.51 2.40 1.05 0.98 1.29 1.15 1.36 1.14 2.56 2.17 2.52 2.67 3.77 3.60 1.33 1.21 1.52 2.81 2.91 3.42 2.68 4.45 5.52 6.59 6.45 6.58 6.83 7.92 7.01 7.75 7.33 7.48 7.73 7.46
Cr2O3 0.00 0.01 0.00 0.01 0.00 0.08 0.10 0.03 0.05 0.02 0.01 0.01 0.02 0.14 0.32 0.01 0.01 0.02 0.01 0.01 0.01 0.01 0.00 0.03 0.07 0.11 0.06 0.09 0.06 0.09 0.03 0.05 0.04 0.15 0.05
FeO 19.19 17.64 18.18 16.38 16.22 6.59 8.44 10.23 11.44 9.86 10.15 11.10 11.83 11.08 10.50 15.11 12.33 14.11 15.27 13.18 16.84 17.12 16.98 13.11 13.68 13.55 13.38 13.76 13.28 14.46 14.22 14.31 13.98 14.22 14.48
MnO 0.29 0.28 0.29 0.27 0.28 0.10 0.16 0.14 0.17 0.22 0.27 0.34 0.26 0.28 0.17 0.69 0.21 0.26 0.49 0.21 0.43 0.29 0.21 0.20 0.19 0.22 0.19 0.20 0.19 0.20 0.24 0.18 0.16 0.17 0.22
MgO 10.86 12.96 12.54 13.35 13.19 20.29 20.25 17.60 16.80 17.83 17.92 17.36 16.97 16.63 16.47 17.13 16.37 15.00 16.09 15.76 13.51 12.91 12.70 14.50 13.80 13.27 13.43 13.38 13.08 13.30 12.95 13.11 13.09 12.57 12.94
CaO 11.06 11.10 11.49 12.17 11.95 12.74 10.87 12.63 12.32 12.39 12.33 11.61 11.11 11.94 12.57 8.17 12.62 12.57 9.09 11.59 10.41 12.31 11.77 11.90 11.81 12.05 11.89 12.16 11.83 11.53 11.94 11.74 11.95 12.03 11.49
Na2O 0.71 0.49 0.58 0.42 0.39 0.14 0.13 0.14 0.16 0.33 0.31 0.46 0.35 0.64 0.20 0.33 0.18 0.16 0.50 0.38 0.78 0.31 0.55 0.94 1.08 0.99 1.07 0.99 1.25 1.37 1.24 1.24 1.15 1.22 1.35
K2O 0.72 0.14 0.23 0.08 0.10 0.04 0.16 0.05 0.03 0.05 0.02 0.04 0.06 0.06 0.05 0.02 0.02 0.04 0.11 0.22 0.06 0.07 0.29 0.37 0.55 0.54 0.55 0.61 0.59 0.50 0.64 0.59 0.61 0.65 0.57
NiO 0.02 0.00 0.02 0.00 0.00 0.00 0.00 0.01 0.04 0.00 0.00 0.06 0.02 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.04 0.07 0.00 0.01 0.04 0.00 0.00 0.01 0.00 0.00 0.03 0.00 0.01
H2O 4.65 4.93 3.96 3.14 3.03 3.17 3.72 3.80 3.76 2.80 3.05 3.33 3.81 3.78 4.49 3.06 3.25 3.61 3.24 3.40 4.05 3.90 3.77 3.94 3.27 4.20 4.30 3.44 3.39 3.59 2.82 3.48 4.25 4.32 4.59
Si 7.396 7.554 7.573 7.845 7.923 7.788 7.810 7.767 7.809 7.649 7.652 7.611 7.627 7.434 7.410 7.824 7.785 7.717 7.605 7.506 7.473 7.505 7.231 7.125 7.007 7.048 7.033 6.978 6.930 6.896 6.859 6.887 6.823 6.815 6.789
AlIV 0.604 0.446 0.426 0.155 0.077 0.212 0.190 0.231 0.191 0.351 0.348 0.389 0.373 0.566 0.590 0.176 0.207 0.263 0.395 0.494 0.527 0.473 0.769 0.875 0.993 0.952 0.967 1.022 1.070 1.104 1.141 1.113 1.177 1.185 1.211
sumT 8.000 8.000 7.999 8.000 8.000 8.000 8.000 7.998 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 7.992 7.979 8.000 8.000 8.000 7.978 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000
AlVI 0.121 0.002 0.000 0.027 0.093 0.003 0.003 0.000 0.005 0.076 0.017 0.037 0.083 0.076 0.028 0.052 0.000 0.000 0.087 0.006 0.072 0.000 0.014 0.085 0.151 0.184 0.189 0.170 0.310 0.125 0.209 0.170 0.142 0.187 0.112
Ti 0.005 0.002 0.003 0.001 0.002 0.021 0.010 0.017 0.025 0.012 0.013 0.017 0.019 0.024 0.081 0.012 0.007 0.024 0.016 0.105 0.022 0.028 0.095 0.127 0.161 0.160 0.159 0.163 0.172 0.173 0.197 0.181 0.194 0.189 0.196
Fe3+ 0.326 0.274 0.204 0.088 0.044 0.123 0.100 0.142 0.096 0.240 0.277 0.309 0.237 0.398 0.342 0.115 0.123 0.123 0.254 0.152 0.398 0.275 0.404 0.320 0.246 0.123 0.150 0.188 0.124 0.310 0.178 0.245 0.282 0.184 0.355
Cr 0.000 0.001 0.000 0.001 0.000 0.009 0.011 0.004 0.006 0.002 0.001 0.001 0.002 0.016 0.036 0.001 0.001 0.002 0.001 0.002 0.001 0.001 0.000 0.004 0.008 0.013 0.007 0.010 0.007 0.011 0.003 0.005 0.005 0.018 0.006
Mg 2.473 2.924 2.816 2.939 2.894 4.255 4.293 3.783 3.631 3.778 3.811 3.715 3.659 3.582 3.575 3.698 3.537 3.288 3.490 3.429 2.991 2.881 2.828 3.189 3.033 2.956 2.989 2.957 2.882 2.946 2.854 2.904 2.923 2.821 2.903
Ni 0.002 0.000 0.002 0.000 0.000 0.000 0.000 0.001 0.004 0.000 0.000 0.006 0.002 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.005 0.008 0.000 0.001 0.005 0.000 0.000 0.001 0.000 0.000 0.003 0.000 0.001
Fe2+ 2.073 1.797 1.974 1.934 1.952 0.589 0.584 1.054 1.232 0.891 0.881 0.914 0.997 0.903 0.936 1.122 1.332 1.563 1.151 1.306 1.516 1.815 1.654 1.267 1.402 1.564 1.501 1.511 1.505 1.435 1.559 1.494 1.451 1.601 1.427
Mn 0.000 0.000 0.000 0.009 0.016 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.003 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000
sumC 5.000 5.000 5.000 5.000 5.000 5.000 5.000 5.000 5.000 5.000 5.000 5.000 5.000 5.000 5.000 5.000 5.000 5.000 5.000 5.000 5.000 5.000 5.000 5.000 5.000 5.000 5.000 5.000 5.000 5.000 5.000 5.000 5.000 5.000 5.000
Mg 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000
Fe2+ 0.052 0.161 0.111 0.000 0.000 0.064 0.321 0.038 0.058 0.041 0.052 0.109 0.197 0.038 0.000 0.592 0.039 0.047 0.453 0.151 0.178 0.052 0.064 0.031 0.039 0.006 0.019 0.006 0.012 0.053 0.021 0.039 0.018 0.006 0.040
Mn 0.038 0.036 0.037 0.025 0.019 0.011 0.019 0.017 0.021 0.027 0.032 0.041 0.032 0.034 0.018 0.085 0.026 0.032 0.061 0.026 0.054 0.037 0.026 0.024 0.024 0.028 0.024 0.026 0.023 0.025 0.030 0.022 0.020 0.022 0.028
Ca 1.810 1.800 1.852 1.925 1.884 1.920 1.655 1.945 1.913 1.887 1.885 1.786 1.722 1.847 1.960 1.267 1.936 1.921 1.417 1.812 1.657 1.911 1.883 1.882 1.866 1.929 1.902 1.932 1.873 1.835 1.890 1.869 1.917 1.940 1.853
Na 0.101 0.003 0.000 0.049 0.097 0.005 0.005 0.000 0.009 0.046 0.031 0.064 0.049 0.081 0.021 0.056 0.000 0.000 0.070 0.011 0.111 0.000 0.026 0.063 0.072 0.038 0.055 0.036 0.092 0.088 0.059 0.070 0.044 0.032 0.079
sum B 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000
Ca 0.000 0.000 0.002 0.000 0.000 0.000 0.000 0.006 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.024 0.059 0.000 0.000 0.000 0.063 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000
Na 0.109 0.140 0.169 0.071 0.014 0.032 0.032 0.039 0.037 0.046 0.054 0.065 0.050 0.096 0.036 0.038 0.050 0.047 0.071 0.095 0.113 0.089 0.133 0.207 0.236 0.247 0.254 0.248 0.266 0.306 0.296 0.288 0.289 0.324 0.316
K 0.140 0.027 0.044 0.015 0.019 0.008 0.030 0.008 0.005 0.009 0.004 0.007 0.012 0.012 0.008 0.003 0.004 0.007 0.020 0.041 0.010 0.014 0.055 0.069 0.103 0.103 0.104 0.116 0.111 0.094 0.120 0.112 0.116 0.125 0.108
sumA 0.249 0.167 0.216 0.086 0.033 0.040 0.062 0.053 0.042 0.054 0.058 0.072 0.061 0.108 0.044 0.041 0.078 0.113 0.091 0.136 0.123 0.166 0.188 0.276 0.339 0.350 0.358 0.364 0.377 0.400 0.416 0.400 0.405 0.449 0.425
total 15.249 15.167 15.215 15.086 15.033 15.040 15.062 15.051 15.042 15.054 15.058 15.072 15.061 15.108 15.044 15.041 15.070 15.093 15.091 15.136 15.123 15.144 15.188 15.276 15.339 15.350 15.358 15.364 15.377 15.400 15.416 15.400 15.405 15.449 15.425
Mg/((Mg+Fe2+) 0.54 0.60 0.57 0.60 0.60 0.87 0.83 0.78 0.74 0.80 0.80 0.78 0.75 0.79 0.79 0.68 0.72 0.67 0.69 0.70 0.64 0.61 0.62 0.71 0.68 0.65 0.66 0.66 0.66 0.66 0.64 0.65 0.67 0.64 0.66
Ca/K 12.901 66.59 41.957 127.77 100.36 248.8 55.647 230.56 369.48 221.48 450.39 243.77 145.81 156.62 234.6 381.21 441.7 285.42 71.991 44.227 159.03 143.59 34.194 27.24 18.136 18.679 18.226 16.639 16.893 19.478 15.736 16.74 16.588 15.496 17.08
ST0226A (leucosome in amphibolite)47 55 54 51 59 61 64
52.75 52.27 49.23 49.22 49.04 48.61 48.22
0.04 0.24 1.18 1.15 1.30 1.28 1.46
2.56 1.93 4.91 4.59 5.04 4.95 5.43
0.01 0.03 0.01 0.01 0.03 0.04 0.04
14.53 12.64 15.25 14.63 14.20 15.06 15.30
0.20 0.24 0.24 0.19 0.17 0.21 0.25
13.81 14.62 14.17 13.84 13.51 13.27 13.50
12.77 12.61 10.95 11.54 11.92 11.60 11.13
0.16 0.26 0.88 0.83 0.70 0.87 1.09
0.08 0.11 0.34 0.33 0.42 0.39 0.39
0.01 0.00 0.00 0.00 0.00 0.00 0.06
3.10 5.05 2.84 3.68 3.69 3.73 3.14
7.729 7.776 7.209 7.273 7.259 7.221 7.117
0.271 0.224 0.791 0.727 0.741 0.779 0.883
8.000 8.000 8.000 8.000 8.000 8.000 8.000
0.171 0.114 0.057 0.072 0.138 0.087 0.061
0.004 0.027 0.130 0.127 0.145 0.143 0.162
0.027 -0.039 0.371 0.285 0.160 0.246 0.343
0.001 0.003 0.002 0.001 0.003 0.004 0.005
3.016 3.242 3.093 3.049 2.981 2.939 2.970
0.001 0.000 0.000 0.000 0.000 0.000 0.007
1.753 1.611 1.348 1.465 1.573 1.581 1.452
0.025 0.031 0.000 0.000 0.000 0.000 0.000
4.998 4.989 5.000 5.000 5.000 5.000 5.000
0.000 0.000 0.000 0.000 0.000 0.000 0.000
0.000 0.000 0.148 0.057 0.024 0.044 0.092
0.000 0.000 0.030 0.024 0.022 0.026 0.031
2.000 2.000 1.718 1.827 1.890 1.846 1.760
0.000 0.000 0.104 0.092 0.065 0.084 0.117
2.000 2.000 2.000 2.000 2.000 2.000 2.000
0.005 0.009 0.000 0.000 0.000 0.000 0.000
0.044 0.075 0.144 0.145 0.137 0.165 0.195
0.015 0.020 0.064 0.062 0.079 0.074 0.073
0.064 0.104 0.207 0.207 0.215 0.239 0.268
15.062 15.093 15.207 15.207 15.215 15.239 15.268
0.63 0.67 0.67 0.67 0.65 0.64 0.66
132.45 98.964 27.051 29.457 24.055 25.112 24.159
M. Calderón, F. Hervé, U. Cordani, H-J. Massonne 261
FIG. 4. Pyroxene Mg-number versus (a) Cr2O3, (b) TiO2, (c) Al2O3, and (d) Na2O.
FIG. 5. Plagioclase Ab-Or-An ternary plots for plagioclases in metamorphosed mafic rocks.
262 CRUST-MANTLE INTERACTIONS AND GENERATION OF SILICIC MELTS: INSIGHTS FROM THE SARMIENTO COMPLEX...
occurs along with chlorite, epidote, titanite and scarceK-feldspar. The plagioclase in the metagabbros ispredominantly bytownite (An73-87; ST0228) andlabradorite (An50-65; ST0229A). Its composition andfresh aspect indicate that this feldspar is a primaryigneous phase. The bytownite composition isconsistent with the primitive character of sampleST0228, which is indicated by the high Mg-numberand Cr2O3 content of the clinopyroxene.
Most plagioclase in the melanosome of theamphibolite is andesine (An37-51). Some is oligoclase(An12-24). The few crystals in the leucosomes thatwere analyzed are mostly andesine in composition(An32-35).
AMPHIBOLE
Metamorphic amphibole in the studied samplesis actinolite and magnesiohornblende (accordingto Leake et al., 1997; Table 3). Actinolite in themafic-extrusive layer is only present in the sheareddomain, where sample ST0204 was collected.Actinolite is an ubiquitous phase in the mafic-felsicintrusive layer. Mg/(Mg+Fe) ratios are significantly
higher in the actinolite in the metagabbros than inthe metabasaltic-andesite (Fig. 6a). This feature isconsistent with the progressive variation in Mg-number of relict clinopyroxene, indicating a bulkcompositional control in the chemistry of actinolite.Some crystals of magnesiohornblende, which areusually replaced by aggregates of actinolite alongcrystal edges, occur in metagabbros.
The amphiboles in the amphibolite, both in themelanosome or leucosome, are magnesiohorn-blende. Those in the melanosome show lowersilica contents (Si ca. 6.8 and 7.0 in tetrahedral sitebased on 23 oxygens; Fig 6a) than in the leucosome(tetrahedral Si 7.1 and 7.8). The composition ofamphibole and plagioclase in granoblastic assem-blages permits the use of the hornblende-plagio-clase geothermometer of Blundy and Holland(1994). Equilibration temperatures of 742±50°Care based on twenty-one pairs of magnesiohorn-blende and andesine plagioclase compositions(Fig. 6b) at pressures between 0 and 2 kbar. Lowertemperatures (645±50°C) were calculated fromfive pairs with oligoclase plagioclase compositions.
FIG. 6. a- Classification diagram of amphiboles(according to Leake et al., 1997); b- Plot ofmean temperature values calculated with thehornblende-plagioclase geothermometer ofBlundy and Holland (1994). The meantemperature for different mineral pairs wascalculated considering pressures of 0 and 2kbar.
M. Calderón, F. Hervé, U. Cordani, H-J. Massonne 263
40Ar/39Ar GEOCHRONOLOGY
FIG. 7. 40Ar/39Ar gas release spectra for selected grains ofmetamorphic amphibole grains from a metagabbro and anamphibolite of the Sarmiento Complex.
Laser heating 40Ar/39Ar analyses of individualgrains of metamorphic amphibole in rocks of the theSarmiento Complex, were carried out in the Centrode Pesquisas Geocronológicas at the Universidadede São Paulo, Brazil. Detailed analytical procedures,including mineral separation, irradiation and massspectrometric measurement procedures are pre-sented by Vasconcelos et al. (2002). Although noneof the analyses allow plateau ages to be calculated,some more or less concordant steps were used tocalculate apparent ‘forced plateau’ ages. Resultsof 40Ar/39Ar analyses are listed in table 4.
The 40Ar/39Ar spectra of an individual amphibolegrain separated from an amphibolite (sampleST0226B), displays little discordance except at onestep in which a fall in the Ca/K ratio is related to anincrease of the apparent age. The low Ca/K ratio ofthe analyzed grain (< 30) is akin to the ratioscalculated for a low-silica magnesio-hornblende inan amphibolite (ST0226A) from the same outcrop.A forced plateau age of 172±5 Ma was calculatedusing four steps of the spectrum (Fig. 7a).
Two amphibole grains separated from a meta-gabbro (sample ST0229A), were analyzed. Bothgrains have Ca/K ratios between 50 and 100, whichare similar to those for some actinolite grains inthe same sample (magnesiohornblende in thissample have Ca/K ratios < 40). The spectrum ofthe first grain displays an old age in the lowestincremental heating step (Fig. 7b). A forced plateauage for this sample that was calculated from threesteps yields an age of 151±12 Ma. The spectrumof the other grain shows less discordance andhas a higher Ca/K ratio. A forced plateau age of143±4 Ma for this sample was calculated usingthe first two steps (Fig. 7c).
The apparent age calculated for an amphibolegrain of the amphibolite is about 20 Ma older thana zircon SHRIMP U-Pb crystallization age near149 Ma for a plagiogranite. This age probablyreflects excess 40Ar in the amphibole grain thatwas incorporated either during amphibolite faciesmetamorphism and/or during the injection andcrystallization of hornblende tonalitic melts. Theages of the actinolite in the metagabbro are con-cordant in part with the U-Pb zircon crystallizationages of the plagiogranite. Taking into considera-tion that perturbation of the K-Ar system in this rock
is low (due to the lack of tonalitic segregations inthe outcrop) and that the age calculated from thegrain with the higher potassium content is moreprecise, an Early Cretaceous (ca. 140 Ma) meta-morphic cooling age is suggested.
264 CRUST-MANTLE INTERACTIONS AND GENERATION OF SILICIC MELTS: INSIGHTS FROM THE SARMIENTO COMPLEX...
TABLE 4. 40Ar/39Ar ANALYSES ON INDIVIDUAL AMPHIBOLE GRAINS FROM A METAGABBRO (ST0229A) AND AN AMPHIBOLITE (ST0226B).
Sample Run ID# Age ± 40/39 38/39 37/39 36/39 40*/39 % Rad(Ma) (Ma)
ST0226B 1734-01A 254.8 5.5 80.5 0.1 18.3 0.1 59.8 73.41734-01B 192.1 3.1 48.6 0.1 15.4 0.0 44.3 90.21734-01C 209.9 7.1 48.9 0.1 14.4 0.0 48.6 98.61734-01D 209.3 13.9 56.1 0.1 15.6 0.0 48.5 85.51734-01E 197.0 15.7 58.7 0.1 18.0 0.1 45.5 76.61734-01F 187.2 4.5 51.2 0.1 14.6 0.0 43.1 83.41734-01G 202.6 5.1 53.4 0.1 16.7 0.0 46.9 86.9
ST0226B 1734-02A 169.5 8.3 66.7 0.1 15.9 0.1 38.8 57.61734-02B 181.0 3.1 43.4 0.1 13.4 0.0 41.6 95.01734-02C 168.3 3.6 39.7 0.1 12.3 0.0 38.6 96.21734-02D 162.6 3.7 40.2 0.1 12.9 0.0 37.2 91.71734-02E 198.7 4.9 42.3 0.1 8.7 0.0 45.9 108.01734-02F 172.0 3.8 40.3 0.1 10.5 0.0 39.4 97.1
ST0226B 1734-03A 270.5 11.9 84.1 0.1 11.1 0.1 63.8 75.31734-03B 194.0 4.2 47.5 0.1 12.3 0.0 44.8 93.51734-03C 178.9 3.1 40.9 0.1 12.1 0.0 41.1 99.61734-03D 167.4 4.0 39.9 0.1 14.8 0.0 38.3 95.11734-03E 240.5 12.6 49.3 0.0 14.9 0.0 56.2 112.91734-03F 182.9 3.6 46.8 0.1 15.0 0.0 42.1 88.9
ST0229A 1733-01A -1,381.5 1731.5 -8.0 0.6 0.0 0.7 -210.9 -1733-01B 0.0 2,5533.7 3,359.7 22.9 155.2 26.3 -4,891.2 -1733-01C 466.3 80.0 245.1 0.1 74.8 0.5 116.3 45.11733-01D 227.4 14.0 67.2 0.0 28.0 0.1 53.0 77.31733-01E 160.3 6.8 46.3 0.1 36.3 0.0 36.6 77.21733-01F 120.3 6.5 39.8 0.1 24.6 0.1 27.2 67.31733-01G 159.5 3.9 41.0 0.1 46.9 0.0 36.4 86.11733-01H 209.6 9.0 46.2 0.1 62.6 0.0 48.6 100.8
ST0229A 1733-02A 148.5 5.2 49.6 0.0 24.6 0.1 33.8 67.11733-02B 141.2 2.9 33.8 0.1 30.7 0.0 32.1 93.01733-02C 172.8 10.1 38.8 0.1 27.4 0.0 39.6 100.21733-02D 88.0 49.3 36.0 0.1 32.7 0.1 19.7 53.51733-02E 142.7 5.3 32.3 0.1 37.3 0.0 32.5 98.11733-02F 136.9 6.4 31.1 0.1 30.8 0.0 31.1 97.9
ISOTOPIC COMPOSITIONS
Sr and Nd isotopic analyses were performed inthe Centro de Pesquisas Geocronológicas(Universidade de São Paulo). Rock samples forisotopic analysis consisted of 1-3 kg of material,which were crushed and split to 200g, and thenpowdered in a tungsten carbide mill. Rb and Srcontents were determined by X-ray fluorescencespectrometry. 87Sr/86Sr ratios were measured bythermal ionization mass-spectrometry, corrected for
mass fractionation to 86Sr/88Sr=0.1194 normalization.Sm and Nd (and other lanthanides) were chemicallyseparated in HDEHP columns supported on teflonpowder. The Sm and Nd concentrations wereobtained by isotopic dilution using a mixed 149Smand 150Nd tracer. The isotopic ratios were calculatedrelative to 146Nd/144Nd=0.7219. All analyses ofradiogenic isotopes were performed using a VG354 Micromass spectrometer. At the time of the
M. Calderón, F. Hervé, U. Cordani, H-J. Massonne 265
analyses the following standard values wereobtained: 87Sr/86Sr=0.71026 ± 0.00002 (2σ) for NBS-987 and 143Nd/144Nd=0.511847±0.00002 (2σ) and0.512662±0.000027 (2σ) values for La Jolla andBCR-1 respectively.
SHRIMP U-Pb geochronological determina-tions on zircons from felsic dikes of the SarmientoComplex and metamorphosed tuffs of the TobíferaFormation show that mafic and felsic magmatismoccurred between 152 and 147 Ma (Calderón et al.,in press). Age dependant isotopic ratios werecalculated at 150 Ma for most samples with theexception of those collected from late (?) plutonicrocks and dikes in the Sarmiento Complex. Forthese rocks, we assumed an age of 140 Ma basedon published radiometric ages (e.g., Stern et al.,1992; 40Ar/39Ar analyses in this study) and fieldrelations. The sample with the lowest 147Sm/144Ndratio (0.064; dolerite) shows the largest variation,one epsilon-neodynium unit (εNd) when calculatedeither at 100 or at 150 Ma. Other samples showvariations of around 0.2 epsilon units. Whole rockSr and Nd isotopic data are listed in table 5 andisotopic relations are plotted in figure 8.
Metasedimentary rocks yielded 87Sr/86Sr initialratios (Sr150; calculated at 150 Ma) of 0.71757(psammo-pelitic schist) and 0.72703 (pelitic schist)and correspondingly low initial εNd150 values of -4.4and -9.1, respectively. Depleted mantle model ages(TDM; after DePaolo et al., 1991) are 1,322 and1,660 Ma, respectively.
Mafic meta-igneous rocks in the SarmientoComplex have relatively depleted isotopic compo-sitions with εNd150 values ranging between +0.8 and+2.3 and TDM between 624 and 706 Ma. The highervalues of εNd150 (+2 and +2.3) correspond to the fine-grained dike of gabbro and the amphibolite. Thepillow basalt yielded Sr150 of 0.70427 and εNd150 of+1.1. The plagiogranite showed a εNd150 value of +2.2.
Coarse and fine-grained granophyres in themafic-felsic intrusive layer yielded more crustal-likeεNd150 values ranging from -5.1 to -5.5 and TDM modelages around 1,300 Ma. The massive dike of allanite-bearing dacite and of brecciated dacite in the mafic-felsic extrusive layer, have lower εNd150 values of -3.9and -2.9, than the granophyres. A foliated metatuff inthis layer yielded a εNd150 value of -5.1 that is indis-tinguishable from the values in the granophyres.
The hornblende dacite yielded a Sr140 of0.704420 and a εNd140 of +0.9, which is similar to that
in the tonalitic leucosome in the amphibolite (+0.8).The hornblende tonalite yielded a Sr140 of 0.70425,a TDM model age of 577 Ma and an εNd140 value of +2.8.Both rocks, which were collected from the mafic-felsic intrusive layer, have similar ratios to the meta-morphosed mafic rocks. The biotite tonalite, in thesouthern tip of the mafic-felsic extrusive layer, has ahigher Sr140 of 0.70451, lower εNd140 +1.0 and slightlyolder TDM model age (724 Ma). The significantlymore primitive igneous rock in the Sarmiento Complexis the dolerite collected from a dike in the maficextrusive layer, which has a εNd140 of +5.4, a TDM of 369Ma and an initial 87Sr/86Sr ratio of 0.70583.
Metatuffs of the Tobífera Formation have crustal-like isotopic compositions with εNd150 varying between-5.1 and -5.4, TDM model ages of 1,139 and 1,169Ma and Sr150 of ca. 0.707 (average value). Thesample with the highest Sr150 (0.7121) is the metatuff,which contains lithic fragments of metasedimentaryrocks among its constituents.
FIG. 8. a- Plot of the initial 87Sr/86Sr ratios versus the values of εNdt
;b- Plot of the 147Sm/144Nd ratio versus the initial 143Nd/144Ndratio. Lithology of samples is indicated in the figure 2.
266 CRUST-MANTLE INTERACTIONS AND GENERATION OF SILICIC MELTS: INSIGHTS FROM THE SARMIENTO COMPLEX...
TA
BL
E 5
. Nd
AN
D S
r CO
MP
OS
ITIO
N F
RO
M S
AM
PL
ES
CO
LL
EC
TE
D IN
DIF
FE
RE
NT
LIT
HO
ST
RA
TIG
RA
PH
IC U
NIT
S IN
TH
E V
ICIN
ITIE
S O
F T
HE
CO
RD
ILL
ER
A S
AR
MIE
NT
O.
Sam
ple
Lith
olog
yAg
eSm
Nd
147 S
m/14
4 Nd
143 N
d/14
4 Nd
(143 N
d/14
4 Nd)
tεεεε ε N
dtT DM
Rb
Sr87
Rb/
86Sr
87Sr
/86Sr
(87Sr
/86Sr
) t
Ma
ppm
ppm
Ma
ppm
ppm
ST03
06P
sam
mo-
pelit
ic s
chis
t15
02.
7112
.78
0.12
800.
5123
450.
5122
19-4
.413
2260
.678
.02.
252
0.72
2369
0.71
7567
FO00
101
Pel
itic
schi
st15
06.
5832
.21
0.12
360.
5121
000.
5119
79-9
.116
6028
7.8
92.9
8.99
50.
7462
100.
7270
30
ST02
-45B
Bas
alt
150
2.90
11.3
60.
1542
0.51
2651
0.51
2500
1.1
774
6.9
20.9
0.95
90.
7063
10.
7042
65
ST03
22B
Fine
-gra
ined
gab
bro
(dik
e)15
01.
856.
700.
1668
0.51
2729
0.51
2565
2.3
659
0.70
5018
ST02
-26B
NM
elan
osom
e (a
mph
ibol
ite)
150
1.12
3.58
0.18
940.
5127
320.
5125
472.
069
2
ST02
-26B
LLe
ucos
ome
(am
phib
olite
)15
05.
1218
.50
0.16
730.
5126
530.
5124
890.
879
6
ST03
22A
Plag
iogr
anite
150
7.76
30.1
70.
1555
0.51
2711
20.
5125
592.
267
5
ST03
37H
ornb
lend
e da
cite
140
1.86
8.91
0.12
640.
5126
190.
5125
030.
979
180
.859
2.7
0.39
40.
7052
040.
7044
20
ST02
-33A
Hor
nble
nde
tona
lite
140
4.15
20.8
50.
1205
0.51
2712
0.51
2602
2.8
577
42.7
804.
30.
154
0.70
4553
0.70
4247
ST02
-50
Bio
tite
tona
lite
140
4.44
22.0
10.
1218
0.51
2620
0.51
2508
1.0
724
60.5
346.
90.
504
0.70
5518
0.70
4514
ST03
29D
oler
ite (d
ike)
140
0.86
8.04
0.06
440.
5127
930.
5127
345.
436
10.
7058
26
ST02
-32A
Gra
noph
yre
150
6.14
30.1
90.
1230
0.51
2282
0.51
2161
-5.5
1309
ST02
-27
Gra
noph
yre
150
6.31
31.1
20.
1225
0.51
2302
0.51
2181
-5.1
1280
ST02
-46
Mas
ive
daci
te (d
ike)
150
5.45
27.7
00.
1189
0.51
2360
0.51
2244
-3.9
1189
130.
835
.610
.635
0.72
9634
0.70
6956
ST02
-45A
Bre
ccia
ted
daci
te15
04.
0721
.11
0.11
650.
5124
100.
5122
96-2
.911
0914
5.4
46.3
9.10
30.
7241
000.
7046
89
ST02
-03
She
ared
fel
sic
tuff
150
6.52
32.0
80.
1228
0.51
2286
0.51
2166
-5.4
1304
74.5
78.0
2.76
60.
7100
200.
7041
22
ST02
-51B
She
ared
fel
sic
tuff
150
2.27
6.91
0.19
880.
5123
700.
5121
75-5
.312
8912
0.0
66.2
5.25
20.
7160
900.
7048
92
ST02
49C
Lapi
lli c
ryst
al f
elsi
c tu
ff15
02.
9315
.34
0.11
530.
5122
960.
5121
83-5
.112
7760
.523
.77.
415
0.72
7890
0.71
2079
ST02
-51C
She
ared
fel
sic
tuff
150
23.4
76.8
0.88
20.
7100
890.
7082
08
ST02
-51G
She
ared
fel
sic
tuff
150
153.
475
.95.
858
0.71
7435
0.70
4944
ST02
-53
She
ared
fel
sic
tuff
150
61.2
32.2
5.51
00.
7165
390.
7047
91
M. Calderón, F. Hervé, U. Cordani, H-J. Massonne 267
DISCUSSION
The lithological, metamorphic and geochemicalvariations in the Sarmiento Complex have beenconsidered to be like those recognized in diverseophiolite complexes (Stern et al., 1976; Stern, 1979;Saunders et al., 1979; de Wit and Stern, 1981). Thedevelopment of secondary phases without schistosityand the downward increase in metamorphic gradefrom zeolite to greenschist to amphibolite facieswithin a variable and short vertical sequence (1-3km) have been associated with processes of seafloorhydrothermal metamorphism (Stern et al., 1976).Greenschist facies dynamic metamorphism and thedevelopment of sheared belts in the SarmientoComplex and Tobífera Formation, are probablyassociated with processes of ophiolite emplacementthat are beyond the scope of this paper. Nevertheless,it is possible that some of the shearing was oceanicin setting.
MANTLE SOURCES AND MAFIC MAGMATISM
Geochemical data from metamorphosed maficrocks of the Sarmiento Complex (Stern, 1979;Saunders et al., 1979; Fildani and Hessler, 2005) putconstraints on compositional features of the mantlesource that produced the mafic magmas. The rare-
earth-element (REE) patterns (mostly mafic lavasand dikes) show LaN/LuN ratios ranging between 1.6and 3.3 after normalization to chondrites (accordingto Sun and Mcdonough, 1989; Fig. 9a). The lack ofheavy REE depletion in the chondrite normalizedpatterns suggests partial melting processes in theupper mantle in the spinel- or plagioclase-lherzolitestability field (P < 15 GPa). Multi-element variationdiagrams for metamorphosed mafic rocks afternormalization to normal-mid-ocean-ridge-basalt(Sun and Mcdonough, 1989; Fig. 9b) display erraticand enriched values in large-ion- lithophile-elements(LILE), such as Rb, Ba and K compared to high-field-strength-element such as Nb and Ta. They alsoshow more or less pronounced Nb-Ta and weak Tinegative anomalies. These compositional featuresare similar to those in continental and island arcbasalts (Pearce, 1982) and suggest that the sourceof the mafic magmas was affected by processesrelated with a supra-subduction setting. A low degreeof partial melting of the mantle sources seems to beneeded. A sign of fluid- or melt-driven metasomatismin the mantle source is the low initial 143Nd/144Ndratios in the metamorphosed mafic rocks that indicatea time-integrated enrichment of Nd relative to Sm inthe mantle source.
FIG. 9. a- Chondrite-normalized REE patterns of metamorphosed mafic rocks and plagiogranite of the Sarmiento Complex. Segmented linerepresents the E-MORB composition; b- Multi-element variation diagram normalized to N-MORB of metamorphosed mafic rocks.Geochemical data from Saunders et al. (1979), Stern (1979) and Fildani and Hessler (2005). Normalizing values are from Sun andMcDonough (1989).
268 CRUST-MANTLE INTERACTIONS AND GENERATION OF SILICIC MELTS: INSIGHTS FROM THE SARMIENTO COMPLEX...
A primitive mantle source is required by theisotopic composition of the dolerite dike. Min-eralogical similarities with the late or post-tectonicdolerite dikes in the metarhyolites of the TobíferaFormation (at Cordillera Riesco) suggest that theserocks belong to a younger episode of mafic mag-matism in which mantle sources and tectonic condi-tions were different. These dolerite dikes could berelated to the Cretaceous mafic magmatism of theBarros Arana Formation, which was derived fromdepleted subcontinental mantle sources (cf. Stern,1991).
Tholeiitic fractional crystallization trends duringmafic magmatism have been indicated by Stern(1979), supported in part by progressive variationsin Mg-number of clinopyroxene (Fig. 4a). Thetitanium-rich clinopyroxene compositions in themetabasalt resemble those in alkaline rocks onclinopyroxene discrimination diagrams [(Ca+Na)versus Ti (not shown); Leterrier et al., 1982].Nevertheless, the high titanium content in this phasecould be controlled by magma chemistry and fastcooling rate at the time of crystallization of basalts (cf.Tracy and Robinson, 1977). High titanium content inthe clinopyroxene is consistent with an evolvedcomposition of the basaltic magma (supported bythe low Mg-number) and quenching processes aresuggested by pillow structures and branching texture.
FELSIC MAGMATISM
The petrological meaning of the granophyreand dacite in the bimodal layers of the SarmientoComplex is a key subject point. Whole rock chemicalanalyses and inherited components in zirconcrystals from granophyres (dated at 147±10 Ma)have been taken as evidence of their derivationfrom melting of continental material bordering themafic complexes into which the mafic magmasintruded (cf. Saunders et al., 1979; de Wit and Stern,1981; Stern et al., 1992). Furthermore, Nd isotopiccompositions of metamorphosed felsic rockssupport the involvement of crustal componentsduring magma genesis.
The plagiogranites, with SiO2 contents between66-75 wt%, relatively enriched and subparallelREE patterns as compared with the metabasalts,and showing Eu depletion (Fig. 9a), have beensuggested to result from extreme fractionalcrystallization of mafic magmas (Stern, 1979).Nevertheless, diverse ophiolite studies haverevealed that the roofs of axial magma chambers
can be marked by polyphase magmatism, partialmelting of amphibolites and intense interactionbetween hydrothermal fluids and rocks (e.g., Gillisand Coogan, 2002 and references therein).Hornblende-plagioclase geothermometry (ca.740±50°C) in an amphibolitic inclusion hosted inthe hornblende tonalite indicates that disequi-librium partial melting (e.g., T > 900°C at P = 0.2GPa; cf. Koepke et al., 2004) and melt segregationprocesses occurred in deeper portions of theophiolite pseudostratigraphy. Therefore, the originof plagiogranites, and probably of the small volumehornblende-bearing lithologies in the SarmientoComplex could involve water-satured melts derivedthrough anatexis of amphibolitized gabbros. Asimilar origin has been proposed for the generationof silicic differentiates in sea-floor spreading centers,such as plagiogranites in ophiolites (Gillis andCoogan, 2002) and silicic volcanic rocks in Iceland(Gunnarsson et al., 1998).
THERMOCHEMICAL MODEL
The integration of energy, species and massconservation into simulations of assimilation, andfractional crystallization processes in complexmagmatic systems is possible with the energy-constrained assimilation fractional-crystallization(EC-AFC) model (Spera and Bohrson, 2001;Bohrson and Spera, 2001). This model follow thethermochemical evolution of an open-systemmagma body through solution of a system of coupled,non-linear ordinary differential equations thatexpress conservation of energy, mass and species(trace elements and isotopes). The thermochemicalapproach used below considers two subsystems(for details see Spera and Bohrson, 2001; Bohrsonand Spera, 2001): a homogeneous magma body ofinitial mass M0 and a finite mass of surroundingcountry rocks. Solution of the EC-AFC providesvalues for the assimilant temperature (Ta), mass ofmelt within the magma body (Mm), mass of cumulates(Mc), mass of anatectic melt assimilated (Ma
*) andconcentration of trace element and isotopic ratios inmelt as a function of magma temperature (Tm). Inputparameters include the equilibration temperature(Teq), the initial temperature and isotopic compositionof pristine magma (Tm
o and εm) and wall rock (Tao and
εa), temperature-dependent trace element dis-tribution coefficients (Dm and Da), heats of transitionfor pristine melt and wall rock (∆m and ∆ha) and theisobaric specific heat capacity of pristine melt and
M. Calderón, F. Hervé, U. Cordani, H-J. Massonne 269
assimilant (Cp,m and Cp,a). Melt-productivity functions[ƒ(Tm) and ƒ(Ta)] exhibit non-linear sigmoidal formsin melt fraction versus temperature at fixed pressureand volatiles fugacity. Phase transition enthalpiesand specific heat capacities are taken from theliterature (Spera, 2000; Bohrson and Spera, 2001;Fowler et al., 2004). The temperature dependenceof the trace-element partition coefficient is estimatedfrom literature. Because the Sm-Nd isotopic systemseems to be unaffected by greenschist andamphibolite facies metamorphism (not the case forthe Rb-Sr system; e.g., DePaolo, 1988), solely theNd isotopic composition of the studied rocks areconsidered in the following discussion.
The thermochemical model for the petrogenesisof the allanite-bearing granophyres and daciticdikes in the bimodal layers (models 1 and 2) requiresthe choice of an appropriate Nd composition repre-sentative of the crustal assimilant at the time of theemplacement of mafic magmas. At the latitudes ofthe Sarmiento Complex, rifting in the Rocas Ver-des basin probably began in the pre-Jurassiccontinental basement rocks of southwesternGondwana. These rocks are represented in thestudied area by the metasedimentary rocks of theStaines Complex. These psammopelitic and peliticschists have εNd150 of -4.4 and -9.1, respectively.These are the most extreme isotopic values (Fig. 8)measured on Paleozoic metasedimentary rockseast of the South Patagonian batholith. Becausethe metasedimentary rocks of the Staines Complex
share common petrographic features, structuresand zircon detrital patterns, with metasedimentaryrocks of the Eastern Andean Metamorphic Complex(cf. Hervé et al., 2003), the correlation of bothcomplexes is suggested. Consequently the avera-ge Nd isotopic composition of existing data(Augustsson, 2003; Calderón et al., 2007) isconsidered representative of the composition ofthe crustal assimilant. The assumed averageisotopic composition of the crustal assimilant usedin the calculation is εNd of -7.3. The composition ofthe fine-grained gabbro in the mafic-felsic intrusivelayer is considered representative of the pristinemafic magmas. Selected results of the EC-AFCsimulation are illustrative of a basaltic magmaintruded at a liquidus temperature of 1200°C (Tm
o)into an upper crust with ambient temperatures of300°C (Ta
o in model 1) and 400°C (Tao in model 2).
These temperatures are within the range expectedin a rift setting. The local liquidus and solidustemperatures of the pelitic crustal assimilant areTs=715°C and Ts=900°C. The equilibrationtemperature Teq is 870°C for both cases. Model 3judges the thermal interaction between the maficmagmas and the host amphibolitic mafic crust inproducing the plagiogranite. In model 3, the isotopiccomposition of the amphibolite is considered to bethat of the mafic assimilant. All input parametersand comprehensive information are listed in table6 and the models are shown by the lines labelledM1, M2, and M3 in figure 10.
FIG. 10. Nd (ppm) versus 143Nd/144Ndresults for EC-AFC models inbimodal meta-igneous rocks ofthe Sarmiento Complex.Dashed lines are EC-AFCmodel paths. The thermochem-ical models for the petrogenesisof the allanite-bearing grano-phyres and dacitic dikes(models 1 and 2; lines labelledM1 and M2 respectively) andplagiogranite (model 3; linelabelled M3) are illustrated. Seetext and table 6 for more details.
270 CRUST-MANTLE INTERACTIONS AND GENERATION OF SILICIC MELTS: INSIGHTS FROM THE SARMIENTO COMPLEX...
The good fit between the actual data and themodels in figure 10 justifies using the calculatedcurves that are labeled M1 and M2 as tentativesolutions to explain the isotopic variations seen inthe mafic-felsic igneous layers of the SarmientoComplex. As shown by Bohrson and Spera (2001),geochemical trends using the EC-AFC modeldisplay non-monotonic behaviours linked to energyconservation and partial melting of the countryrock. After the mafic magma temperature decreasesby 180°C the subhorizontal fractional crystallizationtrend is interrupted and gives rise to a steep mixingcurve between the mafic magma and the radiogeniccrustal rock (Fig. 10). A quantitative evaluation ofthe contamination of the basaltic magma by thecrustal wall rocks indicates a Ma*/Mc (mass ofanatectic melt assimilated/ mass of cumulates)ratio of 0.04 (in model 2; Ta=745°C). Although smallcompositional differences between the basalticlava and dike could be due to source heterogene-ities, the model is in agreement with fractional
crystallization being the main process involved inthe evolution of mafic magmas.
The quantitative analyses of magma contaminationin the granophyres indicate Ma*/Mc ratios between0.63-0.89 at Tm between 895-920°C and with Ta of795-820°C. In the case of the dacites from the mafic-felsic extrusive layer, Ma*/Mc ratios are lower (0.28-0.35), Tm are higher (975-945°C) and Ta are lower(770-780°C). The meta-tuffaceous rocks in the mafic-felsic extrusive layer show similar results to those forthe granophyres. The differences in the degree ofcrustal assimilation among the metamorphosed felsicrocks could be related to different residence times forthe silicic magmas in the magmatic reservoirs,controlled in part by the ease of eruption in responseto the regional stress field.
The results for model 3 for the plagiogranitepetrogenesis are consistent with the fractionationmodel of Stern (1979). They predict the 13%assimilation of the amphibolitic mafic crust (Ma*/Mcratio of 0.13).
TABLE 6. EC-AFC PARAMETERS, GRANOPHYRE AND DACITE (MODELS 1 AND 2) AND PLAGIOGRANITE(MODEL 3), SARMIENTO COMPLEX.
Thermal parameters Model 1 Model 2 Model 3
Magma liquidus temperature (°C), Tl, m 1,200 1,200 1,200Magma initial temperature (°C), Tm° 1,200 1,200 1,200Assimilant liquidus temperature (°C), Tl, a 850 850 1,030Assimilant initial temperature (°C), Ta° 300 400 700Solidus temperature (°C), Ts 715 715 730Equilibration temperature (°C), Teq 870 870 795Crystallization enthalpy (J/kg), ∆hcrys 396,000 396,000 396,000Isobaric specific heat of magma (J/kg per K), Cp,m 1,484 1,484 1,484Fusion enthalpy (J/kg), ∆hfus 270,000 270,000 333,000Isobaric specific heat of assimilant (J/kg per K), Cp,a 1,370 1,370 1,400
Compositional parameters (Nd) Model 1 Model 2 Model 3
Magma initial conc. (ppm), Cm° 6.7 6.7 6.7Magma isotope ratio, εm 0.512565 0.512565 0.512565Magma trace element distributioncoefficient, Dm 0.25 0.25 0.25Assimilant initial conc. (ppm), Ca° 28.4 28.4 3.6Assimilant isotope ratio, εa 0.512071 0.512071 0.512555Assimilant trace element distributioncoefficient, Da 0.25 0.25 0.25Enthalpy of trace elementdistribution coeffcient, ∆Ha -9,000 -9,000
Assimilant in models 1 and 2 is the average composition of metasedimenray rocks (pelite and psammopelitein composition) of the basement metamorphic rocks. Assimilant in model 3 is the amphibolite analyzed inthis study. The composition of the fine-grained gabbro (dike) is considered for the pristine magma.
M. Calderón, F. Hervé, U. Cordani, H-J. Massonne 271
TECTONO-MAGMATIC MODEL
The development of the Rocas Verdes basinprobably involved periods of variable spreadingrates that favored either the extrusion of largevolumes of basaltic rocks or the emplacement andevolution of axial mafic magma chambers. Theconceptual model for bimodal magmatism in theearly tectonic evolution the Sarmiento Complexrequires an early upper crustal magma chamberthat formed by continuous basaltic injection frommantle reservoirs (Fig. 11). The accumulation ofmafic magmas, which probably occurred after aperiod of rapid extension and voluminoussubmarine mafic volcanism, triggered crustalanatexis, segregation and accumulation of silicicmelts with crustal isotopic compositions. Thegranophyres crystallized in the relatively ‘quiet’tectonic zones, whereas the allanite-bearing daciticdikes crystallized from magmas that travelledthrough remnant brittle deformed pathways alongthe rift axis.
A renewed pulse of extension and extrusion ofthe mafic magmas is represented by the dikecomplexes of fine-grained gabbro and basalticflows in the mafic extrusive layer (Fig. 11).Subsequently, remnant gabbroic portions of theophiolitic crust underwent hydrothermal meta-morphism and anatexis during this episode ofenhanced magmatic heat flux from the mantle.Even though there is lack of age control in thehornblende-bearing lithologies in the SarmientoComplex, the above processes must be consideredin the generation of the water-saturated magmas(hornblende-bearing intermediate igneous rocks)along axial magma chambers (Fig. 11).
A magmatic origin similar to that of the allanite-bearing granophyres and dacitic dikes of the Sar-miento Complex is suggested for the metatuffs ofthe Tobífera Formation, which also formed part ofthe seafloor of the Rocas Verdes basin. Thesetuffs share similar ε
Nd150 values (of ca. -5) with thegranophyres. Nevertheless, differences in the Ndand SiO2 contents between the metatuffs (Nd
between 6-16 ppm and SiO2 of 78-81 wt%) and thegranophyres (Nd of ca. 30 ppm and SiO2 of 70-73wt%) suggest that variable thermal regimes ofmagmatic reservoirs, caused by different rates ofmafic magma recharge during magmatic chamberevolution, crustal heterogeneities, and otherfactors, exerted control over processes of assimila-tion and fractional crystallization. The periodicinflux of mafic magmas and their interaction withthe metapelites in the volcanic rifted continentalcrust may represent a first-order heat contributionfor crustal anatexis and silicic magmatism duringthe early Rocas Verdes basin formation.
CONCLUSIONS
According to the discussions and availablegeochemical and isotopic data, the following pointsare worthy of note.
1. Metamorphosed mafic rocks and a plagio-granite from different layers in the ophiolite pseudo-stratigraphy of the Sarmiento Complex are char-
FIG. 11. Hypothetical and schematic model for the petrogenesisof mafic, felsic and intermediate igneous rocks in theSarmiento Complex. The sketch figure (not to scale)shows the different lithologies considered in this study.
272 CRUST-MANTLE INTERACTIONS AND GENERATION OF SILICIC MELTS: INSIGHTS FROM THE SARMIENTO COMPLEX...
acterized by εNd150 values ranging between +1 and+2. Isotopic and trace element compositionssuggest that mafic magmas were derived from anenriched mantle source, previously affected bymetasomatism in a supra-subduction setting.
2. The progressive variation in Mg-number ofthe relict augitic pyroxene records the processes offractional crystallization operating in the maficmagma chambers and along the magmatic/structural pathways.
3. The felsic rocks in the bimodal layers of theSarmiento Complex have crustal-like isotopiccompositions. The εNd150 values of these granophyres
range between -5 and -5.5, whereas those of thedacites are slightly higher (-3 to -4). These valuesexclude their derivation by fractional crystallizationprocesses alone in the mafic magma chambers.
4. Quantitative thermochemical analysesindicate that the felsic rocks formed from magmaswith mass of anatectic melt assimilated/mass ofcumulates ratios (Ma*/Mc) of 0.28-0.35 in the dacitesand of 0.63-0.89 in the granophyres.
5. Model results for plagiogranite petrogenesisindicate that fractional crystallization is the dominantprocess, but also predict the assimilation ofamphibolitic mafic crust by 13% (Ma*/Mc of 0.13).
ACKNOWLEDGEMENTS
This study was financially supported by theConicyt grant and the BAPRTD (M.C), Fondecytgrants 1010412 and 1050431 (F.H.) and Conicyt-BMBF grant to F.H. and H.-J. M. The Álvarez-McCloud family is acknowledged for their logisticaland friendly support during the fieldwork in theremote sampling areas. We would like to thank A.Onoe, L. Petronilho, S. de Souza, H. Sonoki, I.
Sonoki and V. Loios of the Centro de PesquisasGeocronológicas (Universidade de São Paulo,Brazil) who helped with the acquiring andprocessing of the isotopic and geochronologicaldata. T. Theye is acknowledged for his guidanceduring the work with the electron microprobe. Wethank S. Kay, C. Stern, E. Moores and P. Thy for theirconstructive reviews.
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